1. Field of the Invention
The present invention relates to the art of stepped structures, i.e., structures having surfaces which are arranged in repetitions vertical and horizontal offset relationship. More specifically, the invention concerns the construction of stringers for supporting stairways, including spiral stairways, wherein the stringers are formed of a plurality of cast units which are readily assembled in a continuous, stepped relationship.
2. Description of the Prior Art
The erection of custom-made stairways, such as spiral staircases, using metal stringers has been a labor-intensive, tedious and expensive proposition. Typically such erection first involves the separate fabrication of the vertical and horizontal elements from which the stairway stringer is constructed. Using elaborate jigs, these elements are alternately joined together by welding to form a stringer. Only highly-skilled artisans are capable of performing this work with the precision that is required. Even those skilled in the art, however, require a substantial expenditure of their time to construct such stairways. Further, because of the need for special equipment and the lengthy time required to complete a job, it is not practical to perform the work on-site. The work is carried out at the artisan's shop, and those adjustments in the work which often are required because of the peculiar conditions at the site where the stairway is to be installed often do not become apparent until the stairway is put in place and such adjustments difficult to make. To an extent, the inventions disclosed in U.S. Pat. No. 3,474,882 overcome some of the prior art difficulties. That patent discloses constructing a staircase or the like from prefabricated structural elements comprising substantially Z-shaped double angles, each having a rigid central member and a pair of parallel tubular arms connected to said central member near the opposite ends thereof. The arms project in opposite directions from the central member and are adapted to be telescopically connected and secured to one of the tubular arms of another double-angle of the same type so as to form a stairway stringer. U.S. Pat. No. 4,125,175 discloses similar structural elements.
The present invention differs from and possesses several advantages over the structural elements disclosed in the two aforesaid patents. The present invention uses cast rather than prefabricated elements and, consequently, less labor is involved in the forming of the elements. Also the cast structure is more likely to be consistent and uniform from element to element than the prefabricated element which must be welded together whereby errors in alignment can be introduced. From a structural standpoint, rather than being a Z-shaped double angle, as are the elements disclosed in the two patents referred to above, the units of the present invention have both arms, which serve to connect the units to similar adjacent units, extending in the same direction. As a result, according to the present invention, the location where the arms of adjacent units are secured together falls below the adjacent stair plank, whereas the location where the arms of adjacent units are joined falls above the adjacent stair plank in U.S. Pat. Nos. 3,474,882 and 4,125,175.